[llvm] [IR] Initial introduction of memset_pattern (PR #97583)

[Alex Bradbury via llvm-commits]

================
@@ -15230,6 +15230,62 @@ The behavior of '``llvm.memset.inline.*``' is equivalent to the behavior of
 '``llvm.memset.*``', but the generated code is guaranteed not to call any
 external functions.
 
+.. _int_memset_pattern:
+
+'``llvm.memset_pattern``' Intrinsic
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Syntax:
+"""""""
+
+This is an overloaded intrinsic. You can use ``llvm.memset_pattern`` on
+any integer bit width and for different address spaces. Not all targets
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asb wrote:

I've redrafted this, let me know what you think.

https://github.com/llvm/llvm-project/pull/97583